The present invention is particularly of interest in the treatment of titanium or titanium alloy, such as Ti/Al/V alloy, implants, but may also be employed for the treatment of other metallic implants such as those of zirconium and tantalum, and coated metallic implants such as hydroxyapatite-coated titanium.
Dental Implants
Although the instant invention also contemplates orthopedic bone implants, it is equally applicable to dental bone implants.
There are basically two types of dental implants. Those that sit on top of the jaw bone, but under the gums, and those that fit into the jawbone similar to the root of a natural tooth. Each type offers solid, non-mobile support for replacement teeth which act and feel like natural teeth. Since both types are attached to the patient's jawbone they can provide distinct advantages over traditional methods of replacing missing teeth.
There are generally four types of dental bone grafts used: autograflts are those where the bone to be grafted to the jaw is taken, or harvested, from the patient's own body. The area where the bone is harvested from, known as the donor site, is usually the mouth or the hip. This is the paticnt's own bone and is very compatible with the paticnt's body. Autografts are generally the best graft technique and usually result in the greatest regeneration of nissing jawbone. Allografts are taken from human donors. Many countries have donor programs where you can specify that in the event of the patient's death, parts may be harvested from the patient's body to save or improve the life of others. Heart transplants are one type of allograft. This can represent one of the greatest "gifts" you can ever give. Bone obtained in this manor undergoes rigorous tests and sterilization. The patient's body "converts" the donor bone into the patient's natural bone, thereby rebuilding the patient's resorbed jawbone.
Xenografts are harvested from animals. The animal bone, most cormnonly bovine (cow), is specially processed to make it biocompatible and sterile. It acts like a "filler" which in time the patient's body will replace with natural bone. After this replacement process is complete dental implants may be placed to support teeth.
Alloplastic grafts are inert, man made synthetic materials. The modern artificial joint replacement procedure uses metal alloplastic grafts. For bone replacement a man made material that mimics natural bone is used. Most often this a form of calcium phosphate. Depending on how it is made, it may be "resorbable" or "non-resorbable". That is, the patient's body may or may not replace the alloplastic graft with the patient's natural bone. In those cases where it is not replaced it acts as a lattice or scaffold upon which natural bone is built. In either case, the end result is to create enough bone for the placement of dental implants.
There are many implants available, each designed for a specific function. Most are made of titanium, an inert metal which has been proven to be effective at fusing with living bone, a process known as "osseointegration". The cylindrical or screw type implant, called "root form", is similar in shape to the root of a tooth with a surface area designed to promote good attachment to the bone. It is the most widely used design and generally placed where there is plentiful width and depth of jawbone. Where the jawbone is too narrow or short for immediate placement of root form implants the area may be enhanced with bone grafting to allow for their placement.
When the jawbone is too narrow and not a good candidate for bone grafting, a special narrow implant, called "plate form", can be placed into the bone. In cases of advanced bone loss, the "subperiosteal" implant, may be prescribed. It rests on top of the bone but under the gums.
The actual implant procedure involves the surgical placement of the implant or implants, a healing period (osseointegration) and implant restoration to replace the missing tooth or teeth.
The treatment may be a cooperative effort between a surgical dentist who actually places the implant and a restorative dentist who designs, prescribes and inserts the final replacement teeth. Some dentists have advanced training and provide both of these services.
Root form implants are the closest is shape and size to the natural tooth root. They are commonly used in wide, deep bone to provide a base for replacement of one, several or a complete arch of teeth. After application of anesthetic, the patient's dentist will expose the area of the jawbone to be implanted and prepare the bone to accept the implant. The number of incisions and bone preparations depends upon the number of implants (and teeth) being placed. The implant is carefully set into place and the gums are closed with several stitches. The healing period usually varies from as few as three months to six or more. During this time osseointegration occurs. The bone grows in and around the implant creating a strong structural support. In fact, this bond can be even stronger than the original tooth's. When healing is complete, the patient's implant is uncovered and an extension or abutment is attached to it. Now the implant and abutment act as a solid unit ready to support the patient's new tooth or teeth.
Plate form implants are usually used when the bone is so narrow it may not be suitable for the root form implant and the area is not suitable for bone grafting. The plate form implant is flat and long so it can fit into the narrow jawbone. After application of anesthetic, the patient's dentist will expose the area of the jawbone to be implanted and prepare the bone to accept the shape of the implant. The number of incisions depends upon the number of implants being placed. The implant is carefully set into place and the gums are closed with several stitches. Like root form implants, there is usually a healing period for osseointegration, although some plate form implants are designed for immediate restoration.
With very advanced jawbone resorption there may not be enough bone width or height for the root form or plate form implant. In these cases the subperiosteal implant may be prescribed. The subperiosteal implant is custom made and designed to sit on top of the bone, but under the gums. There are two methods for its placement. After application of anesthetic, the patient's dentist will expose the jawbone and take an impression or model of the bone using special materials. This model is used by a dental laboratory to carefully create the custom implant to fit the patient's jaw. A second procedure is then carried out where the jawbone is exposed and the implant placed. The gums are closed with several stitches and replacement teeth are put into place.
For the "single surgery" method the patient's dentist will order a special CAT scan of the patient's jawbone Using the CAT scan data and advanced computer modeling techniques, a model of the patient's jawbone is constructed. This model is used by a dental laboratory to fabricate the custom subperiosteal implant to fit the patient's jaw. A surgical procedure is then carried out where the jawbone is exposed and the implant placed. The gums are closed with several stitches and the replacement teeth are put into place.
Bone Implants
Owing to the rapid development of surgery, it is nowadays possible to carry out operations to bones and joints which were still inconceivable a little while ago. For example, it is now possible to carry out surgical removal of cysts, foci of suppuration in bone and malignant tumors from bones. This results in defects in the bone, which need to be filled since normal bone repair processes are no longer able to compensate them. Some defects of this type may have a volume of up to 600 cm.sup.3 which has to be filled again.
For filling cavities of this type use is made of bone replacement materials in liquid, pasty or solid form as granules or articles for implantation. If the cavities which are to be filled are not too large then the purpose of the bone replacement materials is to temporarily fill the cavities in the bone and to allow the body itself to compensate, in the course of time, the defect with living bone material.
This may entail there being either growth around the replacement material, which stays in place without irritation, or slow breakdown and replacement thereof by living bone.
It is necessary to use a material which is compatible with bone for filling larger cavities with bone replacement material. The materials of this type which are used are endogenous or exogenous fragments of bone or bydroxyapatite granules. Only very limited amounts of endogenous bone material are available, and additional surgical operations are necessary to obtain it. It is necessary to remove all antigens from exogenous, for example animal, bone materials in order to avoid rejection reactions, but this is only partially successful in practice.
When hydroxyapatite is used there is primarily irritation of the surrounding bone material. When this occurs a material in the form of a liquid, pasty or solid can be used for filling cavities in bones, that is to say for filling bone defects and which does not cause any primary irritations.
Bone implants are frequently used in surgical operations. Bone implants are items which are implanted in the bones of the body of a recipient and permanently replace parts of the skeleton or roots of teeth. The outer layer of the bone implant, which comes into contact with the living substrate bone, is termed the bone-contact layer. At the present time, metals, such as, for example, special steels, noble metals, titanium, ceramic materials, such as, for example, alumina, glass-eramics, hydroxy-apatite ceramics and synthetic materials are used as bone implants and as bone-contact layers.
These substances are classified as biocompatible and bioactive according to the tissue compatibility. Biocompatible substances are tolerated by the body in the long term without rejection. Bioactive substances become rigidly incorporated like endogenous tissue, the tissue compatibility being determined by the chemical composition, the crystalline structure, the surface structure and the mechanical properties.
The metals and some ceramic materials, such as, for example, alumina ceramics, are biocompatible. Ensheathing by connective tissue always takes place in the body. This connective tissue layer allows the implant to be held relatively rigidly, but does not allow frictional connection to the mineral framework of the substrate bone.
Because of the absence of primary integration into the substrate bone, a biocompatible implant of this type can be exposed to only slight mechanical stress since otherwise it is held increasingly poorly, and this is associated with pain and, finally, the loss of the implant. This is found, for example, with hipjoint prostheses, which are always subject to great stress and for which nowadays more than one quarter of the operations are carnied out because of loosening of an implant which had previously been inserted.
Thus, additional undercutting such as, for example, a screw thread is necessary for permanent mechanical anchoring of biocompatible implants in bone. With all metallic implants it is still an unanswered question of whether they release toxic metal ions into the surroundings and thus may have adverse effects in the long term.
Even when bone cement is used, despite the initially better mechanical connection to the substrate bone, a loosening which has been described takes place, with some delay.
In the case of bioactive materials, after some time the bone material grows directly on such materials. Among the known materials, the best properties in this respect are shown by hydroxyapatite which, after a period, which lasts only a few weeks, of mild signs of irritation, which can be detected under the microscope by giant cells around the implant, is integrated into the substrate bone without an interlayer.
As a general rule, bioactive materials are difficult to work and less mechanically stable than the biocompatible metals or ceramics.
Thus, there has been a shift by the medical profession to the use of combined implants comprising biocompatible cores such as, for example, titanium, special steel and alumina, and bioactive surface coatings (compare German Patent Specification No. 2,840,064). A combined implant construction of this type may have considerable advantages since, in this case, high mechanical stability of complex-shaped implants is combined with rapid and rigid connection to the substrate bone. As indicated above, signs of irritation appear after the insertion of implants of this type with the bioactive coatings hitherto known, but these signs subside after some time. Thus, there exists a great need for a treatment which can be used to promote the production of new bone tissue and accelerate the healing process around such implants
It is contemplated by the present invention that any or all of the foregoing implants may include areas of micro-geometry as disclosed in the copending application Ser. No. 08/996,244, filed Dec. 22, 1997; all valid portions thereof, incorporated herein by reference.